The invention relates to a device for generating a low-consumption controlling analogue voltage of a stable value, more specifically intended for controlling multi-input matrix circuits such as driver circuits for LCD liquid crystal displays, these circuits also being known as LCD xe2x80x9cscreen driversxe2x80x9d.
Turning to FIG. 1 illustrating the prior art, the driver systems for liquid crystal displays comprise a controller circuit CC driven by a microprocessor xcexcP. This controller circuit CC has a controller strictly speaking C and a charge pump enabling control voltages of a higher amplitude, which may reach 9 V, to be generated from a supply voltage of a standard value. The control voltages are applied in the form of rectangular voltages of a given amplitude, 1.8 V, and switched between the ground voltage and different successive levels up to the maximum voltage applied by the charge pump, 6 levels of 0 to 9 volts by steps of 1.8 volts, these rectangular voltages of different levels in effect enabling the contrast level to be regulated depending on the address of the LCD segments controlled.
However, because of the relatively high capacitance, 200 pF, of the LCD segments, it is necessary to provide external capacitance, the purpose of which is to smooth the voltages finally applied. In spite of incorporating the above-mentioned capacitance, it nevertheless remains difficult to specify accurately the voltage levels applied to the LCD, leading to a degradation in the contrast finally applied, especially when dealing with the highest voltage values. The bridge divider becomes imbalanced as soon as a current is applied on one of the intermediate levels of the latter, since this current charges the capacitance of the LCD segments.
One solution to reducing the relative variations in contrast applied might be to use resistances of a lower value at the output of the controller circuit, which, by increasing the value of the current, will enable the relative variation in contrast to be reduced.
However, the solution outlined above has a major disadvantage in that it causes too high a current to be applied on the charge pump, making it necessary to increase the size of the charge pump and the external capacitance.
The objective of this invention is to overcome the disadvantages and limitations of the driver circuits used for LCD screen displays by employing a device for generating a low-consumption controlling analogue voltage of a stable value.
Another objective of this invention is to increase by a factor of at least 75 the autonomy of on-board or portable computer systems provided with LCD liquid crystal display screens in pilot mode, using a device for generating a low-consumption analogue controlling voltage having a stable value.
Another objective of this invention is to eliminate the external capacitance at intermediate levels of the bridge divider, which, by integrating the latter, will lead to a decrease in the number of inputs outputs and a reduction in the size of the chip.
Yet another objective of the present invention is to use a smaller size charge pump due to the very low consumption of the system as a whole, it being possible to eliminate the external capacitance on the charge pump and reduce the size of the chip accordingly.
Finally, another objective of the present invention is to reduce integration costs and achieve low consumption, whilst increasing the autonomy and accuracy of the display.
The device used to generate a low-consumption controlling analogue voltage of a stable value from an analogue voltage of a given nominal value, proposed by the invention, is remarkable due to the fact that it comprises an input circuit receiving this nominal value analogue voltage enabling a picture analogue voltage to be generated at a value reduced in a given ratio k. Furthermore, a driver circuit receives this picture analogue voltage as a reference value and a picture signal of the controlling analogue voltage, the picture signal being formed by this controlling analogue voltage reduced in the same given ratio k. This driver circuit has at least one differential amplifier supplied by a first constant voltage of an amplitude higher than the maximum value of the picture analogue voltage and by a second constant voltage of a given amplitude and outputs a first switch control pulse, synchronous with the reference signal and of a lower amplitude than the first constant voltage, and a second switch control pulse, synchronous with the reference signal but complemented with regard to the first control pulse. A switching circuit for the controlling analogue voltage, supplied by the analogue voltage of a given value, is provided, this switching circuit having at least a first switching branch modelled as an inverter/amplifier, controlled by the first switch control pulse and outputting an amplified auxiliary switch control pulse, synchronous with the reference signal, and a second switching branch, modelled as an inverter/amplifier, controlled by the amplified auxiliary switch control pulse and by the second switch control pulse and outputting the switched controlling analogue voltage at the analogue voltage of a given nominal value.
The device for generating a low-consumption, controlling analogue voltage of a stable value, proposed by the invention, finds application in driver circuits which use control signals of the stepped type, for example circuits such as those used for LCD display screens, particularly if these devices are provided in the form of integrated circuits in CMOS technology.